Common techniques for fabricating displays and semiconductor electronic devices involve several imaging steps. Typically, in each step, a substrate coated with a resist or other sensitive material is exposed to radiation through a photo-tool mask to effect some change. Each step has a finite risk of failure. The possibility of failure at each step reduces the overall process yield and increases the cost of the finished article.
A specific example is the fabrication of color filters for flat panel displays such as liquid crystal displays. Color filter fabrication can be a very expensive process because of the high cost of materials and low process yield. Traditional photolithographic processing involves applying color resist materials to a substrate using a coating technique such as spin-coating, slit and spin or spin-less coating. The material is then exposed via a photo-tool mask and developed.
Direct imaging has been proposed for use in the fabrication of displays and in particular color filters. A color filter substrate, also known as a dye-receiving element, is overlaid with a dye donor element. The dye donor element is image-wise heated to selectively transfer a dye from the donor element to the receiving element. Image-wise heating is typically done by means of a laser beam. Diode lasers are particularly preferred for their ease of modulation, low cost and small size.
“Dye transfer” processes are a particular type of “thermal transfer” process. Other thermal transfer processes include: laser-induced melt transfer, laser-induced ablation transfer, and laser-induced mass transfer. The term “thermal transfer process” is not limited to the image-wise transfer of dyes. As used herein, the term thermal transfer process includes the image-wise transfer of donors coated with pigments or other colorant compositions.
Direct imaging systems typically employ hundreds of individually modulated beams in parallel to reduce the time taken to complete images. Imaging heads with large numbers of such “channels” are readily available. For example, one model of SQUAREspot® thermal imaging head manufactured by Kodak Graphic Communications Canada Company, British Columbia, Canada has 960 independent imaging channels, each channel having power in excess of 25 mW. The array of imaging channels can be controlled such that an image is written in a series of swaths which are closely abutted to form a continuous image.
One problem with multi-channel imaging systems is that it is extremely difficult to ensure that all channels have identical imaging characteristics. Different imaging characteristics among channels may result from differences in the output radiation that the channels project upon the imaged media. Variations in the output radiation emitted by the array of imaging channels may originate from channel-to-channel variations in power, beam size, beam shape and/or focus. These variations contribute to the production of a common imaging artifact known as banding. Banding is often particularly prominent in the area between two successively-imaged swaths. This is primarily because the end of the last imaged swath and the beginning of the next imaged swath are usually written by channels at opposite ends of a multi-channel array. As such, these channels are more likely to have differing imaging characteristics. A gradual increase in spot size from channel-to-channel may or may not be visible within the swath itself, but when a swath is abutted with another swath, the discontinuity in spot size at the swath boundary may result in a pronounced artifact in the image. Banding can be a function of any overlap or separation of successive swaths as well as channel variance within each of the respective swaths.
Accurate swath positioning is necessary for mitigating banding but, in itself, cannot fully eliminate banding. Any remaining levels of banding therefore need to be reduced or masked by adjusting the channels within the swath itself. This can require a careful alignment and calibration of the channels within the array. One could attempt to establish a uniform power distribution across all the channels in the imaging array to minimize inherent channel-to-channel differences. However, because of the role of other contributions mentioned earlier (e.g. beam size, beam shape, and focus) power uniformity across the imaging channels of the array does not guarantee reduced banding.
Banding is not solely attributable to the imaging system. The imaged medium may also contribute to banding. Various methods for evaluating banding involve inspection or characterization of the imaged medium.
Several problems may arise when a thermal transfer process is applied in the production of color filters. Some common color filters have a repeating pattern of spaced-apart color element lines. Each of the lines corresponds to one of three colors. Each of the lines is typically smaller in width than the swath imaged by the imaging head. Consequently, inter-swath banding may result wherein varying color transfer efficiency causes variations between different ones of the color element lines, as well as within individual lines. Since the lines form a repeating pattern, a visual beating readily perceptible by the human eye may result, consequently reducing the quality of the color filter.
Some prior art multi-channel imaging systems have employed calibration methods that attempt to create uniformity in the output radiation emitted by all of the imaging channels in the array, or alternatively, to create uniformity in the optical properties across the entire width of a swath in an image that has been imaged by all of the imaging channels in the array. The inventors have determined that such calibration methods are not typically well suited for imaging repeating patterns of features in which inter-swath banding is present since each of the features are smaller in width than the width of the imaged swath.
Patents and patent applications in the field of imaging include:
U.S. Pat. No. 4,965,242De Boer et al.;U.S. Pat. No. 4,804,975Yip;U.S. Pat. No. 6,146,792Blanchard-Fincher et al.;EP 434,449Sprague et al.;U.S. Pat. No. 6,832,552Patten et al.;U.S. Pat. No. 5,546,165Rushing et al.;U.S. Pat. No. 6,618,158Brown et al.; andU.S. Pat. No. 5,278,578Back et al.
There remains a need for imaging methods that lessen the visibility of swath-to-swath and inter-swath banding, especially in the imaging of regular patterns such as required by color filters.